For a variety of reasons large pulverized coal fired boilers are increasingly bearing the burden of frequent load swings. The resulting variation in operating levels has increased the operation of these boilers under low load conditions. This consequently heightens the need for a burner capable of a reliable, efficient, low load performance that still enables NO.sub.x formation to be kept to an acceptable minimum level. A key factor which increases NO.sub.x formation is the oxygen available in the combustion zone immediately downstream of the burner nozzle.
Typical burner nozzles such as those described in U.S. Pat. No. 4,497,263 issued to Vatsky et al. and U.S. Pat. No. 4,457,241 issued to Itse et al. are of the type where the pulverized coal particles are concentrated into the center of an air-coal stream before these particles are burned in the boiler. This method, although sufficient for the burning of the pulverized coal, contributes to NO.sub.x formation because of the oxygen available during combustion.
Another factor influenced by burner nozzle performance is the stability of the flame. The velocity of the fuel emerging from the nozzle is of prime importance to flame stability. Lower fuel velocities provide more time for the particles to heat up and ignite in the burner throat and thereby achieves a more stable flame. Difficult to ignite fuels such as low volatile coals particularly benefit by lower fuel velocity. Lower velocities also limit air-fuel mixing prior to burning which reduces the availability of oxygen during combustion thereby reducing NO.sub.x formation.
It is thus an object of this invention to provide a burner nozzle that is efficient to operate with difficult to ignite fuels and one which reduces NO.sub.x formation. It is another object of this invention to introduce a low velocity fuel mixture into the furnace of a boiler for improved ignition performance and to reduce the oxygen available during initial combustion so as to reduce NO.sub.x formation. A further object of this invention is to provide a burner nozzle which increases flame stability and one which is easily capable of being retrofitted into existing burners. Another object of this invention is to separate the pulverized coal into a relatively fuel-dense low velocity stream and a relatively fuel-dilute high velocity stream with low pressure loss across the nozzle.